1. Field of the Invention
The present invention concerns a device for measuring parameters, in particular parameters relating to aircraft or vehicle wheels, such as tire pressures, for example, specifically a device of the type comprising at least one sensor, on a wheel for example, an electronic circuit associated with the sensor, means for transmitting signals conditioned by the measured parameter values and calculator means receiving and processing the measured value signals.
2. Description of the Prior Art
In devices for detecting parameters such as aircraft wheel tire pressures, for example, the measured value is converted by electronics associated with the sensor into a signal which is sent via coupling means between the rotating part and the fixed part of the wheel to means such as a computer, for example, for processing the information embodied in the signal. One of the main problems experienced with such devices arises from the effect of certain kinds of interference on the sensor whereby the measurement is subject to an offset or a drift. In the case of these pressure sensors the main interference factor is the temperature, variations in temperature causing sensor drift, although other forms of interference such as vibration or electromagnetic fields may be operative simultaneously, of course.
To compensate for temperature dependent sensor drift provision has been made to associate with the sensor on or in the wheel electronic means for compensating sensor drift caused by temperature variations so that the coupling means transmit a signal which is as close as possible to the theoretical signal representing the measurement in the absence of any interference, the signal passing through the coupling means (such as a rotary transformer, for example) and being then sent to the computer for processing.
For reasons connected with manufacture and maintenance, and in order to site the electronic means as close as possible to the physical area in which the sensor is disposed, provision has been made for integrating the sensor and its electronics into a common casing mounted in the tire, the purpose of the electronic circuit being on the one hand, to compensate for sensor drift and, on the other hand, to convert the signal into a form suitable for transmission by the coupling means.
The solutions currently used have serious drawbacks, however. Given, on the one hand, the accuracy of measurement that is now required and, on the other hand, the range of variation of the interference factors affecting the sensor, for example a very wide temperature range (from -55.degree. C. to +180.degree. C.), it becomes necessary to use extremely high-performance sensors whose drift is as regular as possible and in particular is proportional so that it can be accurately compensated by the electronic means over all the range of variation of the measurements and irrespective of the level of interference. Such sensors are naturally costly. Further, the associated electronic circuit which shapes and transmits measurement signals must itself be highly sensitive and of high quality, further increasing the unit cost of the system.
Of course, the difficulty increases with the performance of the installation incorporating the sensors, for example with the performance of tires which are specially designed to withstand higher temperatures, or when a second and even a third interference factor is operative.
In some cases there is provision for digital processing of pressure and temperature signals from a pressure sensor, carried out in a microprocessor, for example, using equations of the characteristic curve of the sensor obtained by calibration of the sensor over all of the range of temperature variation, these equations being stored in a memory associated with the microprocessor. This kind of implementation requires lengthy and costly calibration and uses a lot of memory capacity.
The present invention proposes to remedy these drawbacks and to provide a device for measuring parameters, in particular parameters relating to aircraft or vehicle wheels, which is of extremely reduced unit cost and simple to maintain although it is highly reliable and is able to provide measurements of sufficient accuracy over extremely wide ranges of variation in the interference factors, whilst also enabling the use of interchangeable sensors, and even replacement of sensors by sensors of a different type, all of this combined with a reduced memory capacity requirement.